In a typically anti-American sermon, he restated his demand that US troops leave Iraq.

He also called Israel, Britain and the US the “evil trio”, in an echo of George Bush’s “axis of evil” comment.

The US said Mr Sadr fled to Iran at the beginning of its troop “surge”, designed to restore security and order in Baghdad. Aides to the young cleric, who led two uprisings against US forces in 2004, insist he never left the country.

Mr Sadr’s public reappearance comes at a crucial moment. President Bush, who has just won fresh funding for the war without having his hands tied over a withdrawal date, yesterday predicted that summer would be a critical time for US forces.

Mr Bush told a news conference in Washington he expected heavy fighting in Iraq in the weeks and months ahead, and predicted insurgents and al-Qaida would try to influence US public opinion by launching major attacks.

In September, the US commander in Iraq, General David Petraeus, is to report back to the White House on the progress of the new US strategy.

In approving new funds for the war yesterday, Congress set down “benchmarks” that the Iraqi prime minister, Nuri al-Maliki, has to meet on measures to promote national reconciliation, such as a new law on dividing oil revenues.

But six Sadrist ministers – Mr Maliki depends heavily on Mr Sadr’s support in parliament – withdrew from the weak and fractious government last month in protest at the prime minister’s refusal to set a timetable for a US troop withdrawal.

The main Sunni Arab bloc in the government has also said it is considering pulling out over what it says is growing sectarian violence and rampant corruption.

After a relative lull, sectarian murders – from Sunni suicide bombers and retaliatory killings from Shia death squads – have been on the rise again, indicating that the Mahdi army militia may be becoming more active. The militiamen have largely stayed off the streets since the US crackdown started.